1. Field of Invention
The present invention relates to the recovery of petroleum fluids from an underground reservoir. It relates primarily to tertiary recovery of oil, i.e., the recovery of the oil remaining after waterflood operations.
2. Setting of the Invention
As is well-known, petroleum is found in reservoirs ranging from shallow to very deep and is produced by drilling boreholes into the reservoir. The first stage of producing oil from an underground reservoir is commonly referred to as the primary production stage. During this period of time, oil flows into the wellbore under natural forces, and if the natural forces are sufficiently great, the oil flows to the surface through the wellbore where it is recovered. After the natural pressures have been depleted to a certain extent, it may be necessary to put pumps in the wellbore to pump the fluid from the wellbore to the surface. By this method of production, it is normally possible to obtain only from about 5% to 15% of the oil originally in-place. After the primary production has been depleted, it is a common practice to institute secondary recovery operations. One of the more common secondary recovery operations is the so-called waterflooding technique, wherein water is injected into injection wells and drives oil toward a second set of wells, called producing wells. This can sometimes recover an additional 50% maximum of the original oil in the reservoir. Even after waterflooding, there remains a large part of the original oil still unproduced and remaining in the reservoir. This could be up to 30%, 40%, or even 50% or more of the original oil in-place. Tertiary recovery methods have been developed and are being developed in an effort to obtain a good part of this remaining oil. A most common method is where a fluid miscible or partially miscible with the reservoir oils is injected through injection wells to drive oil to the producing wells. A major problem in tertiary recovery is that all reservoirs are heterogeneous, i.e., there are some streaks that are more permeable to the flow of fluid therethrough than other portions of the reservoir. Thus, when an oil recovery driving fluid is injected through the wells, it tends to go through these more permeable zones, bypassing the less permeable zones and leaving significant amounts of oil therein. One method of trying to reduce this "channeling" or breakthrough process that has been proposed is to inject mobility-reducing fluids through the injection wells prior to injecting the miscible fluid in an effort to partially block flow in the more permeable zones of the reservoir. This latter procedure normally tends to greatly reduce the injectivity of the injection wells, i.e., the oil recovery fluid cannot be injected at nearly as high a rate as before the mobility-reducing agent was injected therein.